Sensor array having a pulse echo radar

ABSTRACT

A sensor array with a pulse echo radar system with which a carrier signal of a microwave transmitter oscillator ( 2 ) is transmitted in the form of pulses with a specified pulse repetition frequency in a transmission window. This radar signal is reflected from a target object and the position of the target object is calculated in a mixer ( 7 ) from the times of transmission of the pulse to the arrival of the reflected radiation. An array of several transmitter and receiver units ( 11, 20, 30, 40 ) with switches (R 1 , R 2 , R 3 , R 4 ) can be constructed in which the stochastic pulse sequences of each transmission window in each receiver branch ( 7, 9, 10 ) are known and the transmitter and receiver units ( 11, 20, 30, 40 ) are linked to one another in such a way that in each of the receiver branches ( 7, 9, 10 ) the stochastic pulse sequences of each transmission window are detected separately and thereby the cross echoes can also be analyzed.

BACKGROUND INFORMATION

[0001] The present invention relates generally to a sensor array having a pulse echo radar.

[0002] It is known, for example from German Patent Application No. 44 42 189 A1, that sensors having transmitting and receiving units for both sending and receiving data are used in a system for measuring distance in the area surrounding motor vehicles. With the aid of the distance measurement, passive measures to protect the vehicle can be activated, for example in the event of a head-on, side, or rear-end collision. With an exchange of the data that is acquired, an evaluation of the traffic situation can be carried out, for example, for the activation of appropriate deployment systems.

[0003] In addition it is also generally known that distance can be measured using a pulse radar in which a carrier pulse having a rectangular envelope of an electromagnetic oscillation is transmitted in the gigahertz range. This carrier pulse is reflected from the target object and the target distance and, with limitations, using the Doppler effect, also the relative speed of the target object can easily be determined from the time of transmission of the pulse to the time of arrival of the reflected radiation. A measurement principle like this is described in A. Ludloff's technical book, “Handbuch Radar und Radarsignalverarbeitung” [Handbook of Radar and Radar Signal Processing], pages 2-21 to 2-44, Vieweg Verlag, 1993.

[0004] A plurality of radar sensors for the individual conflict situations in the area surrounding the motor vehicle are needed as a rule for the reliable deployment of the passenger protection system in a motor vehicle that was mentioned at the outset. For example, early collision recognition (precrash recognition) is needed to permit timely detection of an object that constitutes a danger for the vehicle's passengers in a collision.

[0005] In this regard, the known radar sensors for pulse echo operations as a rule transmit the pulses with a fixed repetition frequency, so that the pulse trains from different transmitters thus generally cannot be distinguished. When the transmitters of a microwave radar system operate independently from one another, e.g., with a pulse repetition frequency of 6 MHz, a carrier frequency of 24 GHz, and a pulse width of approx. 350 ps, each sensor receives only object echoes of its own pulses, while the echoes of pulses from the other transmitters are either not received or received only in the form of signal noise.

[0006] It is known per se, for example from German Patent Application No. 198 02 724 A1, that in a monitoring device coded ultrasound pulses are transmitted that are detected with appropriate signal echo sensors. In this regard, for the sake of differentiating the pulses that are received, they are coded previously by means of a random generator and then selectively analyzed accordingly.

SUMMARY OF THE INVENTION

[0007] The present invention is based on a sensor array having a pulse echo radar system as mentioned at the outset in which a carrier signal of a microwave transmitter is transmitted in the form of pulses with a specified pulse repetition frequency. This microwave signal is reflected from a target object and the position of the target object is calculated in a mixer circuit from the time of transmission of the pulse and the arrival of the reflected radiation. In the process, the pulses are transmitted in an advantageous manner only with a predefined probability by means of a random switch and then the stochastic pulse sequences of the transmitter are known at the receiver for analyzing the signals that are received. Through this coding the differentiation of several transmitted signals is also possible.

[0008] This and other features of preferred further embodiments of the invention are presented in the following description and the drawings, with each of the individual features capable of being implemented by itself or severally in the form of subcombinations in the embodiment of the invention also in different fields and can represent embodiments that are advantageous and also inherently patentable for which protection is claimed here.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The sensor array according to the invention is described with reference to the drawings of advantageous exemplary embodiments wherein:

[0010]FIG. 1 shows a schematic block diagram of a transmitter and receiver unit of a stochastically coded microwave radar system using the pulse echo process, and

[0011]FIG. 2 shows a schematic block diagram of an array of transmitter and receiver units of a microwave radar system according to FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0012] According to the invention, in a preferred embodiment an array is constructed from several transmitter and receiver assemblies which are present in any number n≧1, in which the stochastic pulse sequences of each transmitter are known at each receiver and the transmitter and receiver units are linked with one another in such a way that the stochastic pulse sequences of each transmitter can be analyzed separately at each receiver. With the circuit arrangement that was previously mentioned, the transmission window can be enabled as a function of the stochastic pulse train of the respective transmitter, and a receiver window is opened at each receiver for all pulses that are transmitted. A correlation of the signals pertaining to each transmitter with the respective signals that are received is carried out in an advantageous manner after the signals that have been received are mixed with the carrier signal in each of the respective mixer circuits of the transmitter and receiver units.

[0013] With the stochastic coding of the transmission pulse trains according to the present invention it is possible in principle to reconstruct the transmitter and transmission time from a pulse train that is received using an array of several transmitter and receiver units and thus increase the immunity from interference by other transmitters, since the coding affects the detection along with the reception interference power. Thus a cross-echo analysis of the whole array for determining the location of the target object is made possible.

[0014] Furthermore, late echoes can be discriminated, i.e., of echoes of objects that lie outside the area of unambiguity (for example, 15 m when used in the automobile industry). This also applies to apparent objects, which appear to lie outside the area of detection due to multiple reflections. In a simple manner, an even higher signal-to-noise ratio can be achieved if the pulse repetition frequency is increased, in which case the inter-pulse period may even fall below the light transit time over the detection area.

[0015] In an advantageous embodiment, each of the transmitter and receiver units has a FIFO register controlled by the switch in the transmission branch of the respective transmitter and receiver unit with which each received signal is switched to a separate accumulator for correlation of the signals pertaining to each transmitter with the respective signals received with a delay.

[0016] In FIG. 1 a schematic block diagram of a transmitter and receiver unit 1 of a microwave radar system is depicted, which has a transmitter oscillator 2 for the carrier signal, e.g., with a frequency of 24 GHz, the first output of transmitter oscillator 2 being switched via a switch 3 in such a way that there is a pulsed output signal at output 4 of unit 1. Switch 3 is controlled by a pulse generator 5, which may have a pulse frequency of 50 MHz, for example. The timing signal of pulse generator 5 is sent here via an additional switch R₁ through which the transmission pulse sequence is stochastically coded.

[0017] In this process switch R₁ ensures that not every pulse from pulse generator 5 is transmitted but rather that a pulse is only sent with a probability P prescribed by switch R₁. The received signal reflected from a target object arrives at a receiver input 6 of unit 1 and is sent to a mixer 7. The carrier signal of transmitter oscillator 2, which is pulsed in the same way via a switch 3′ with the coded timing signal of pulse generator 5, albeit delayed by a time factor τ (0 to 2π), is also sent to this mixer 7.

[0018] In a manner essentially known from the related art, a distance window of the sensor array is defined with the delay in unit 8, since the signal that is transmitted, reflected from the target object, and received at input 6 also has a transit time that is determined by the distance of the target object. The signal that depends on the distance and is obtained through the mixing is sent to an accumulator 10 after filtering and absolute-value generation in a low pass filter 8. For the pulse echoes reflected from the target object only a fraction of the energy in accumulator 10 that would have been received without coding will be detected because of the coding if the correct pulse delay has been selected. This effect can be compensated for through a simple increase in the pulse repetition frequency from, e. g. the usual 6 MHz to 18 MHz or higher.

[0019] From FIG. 2 one can perceive the design of an array of transceiver units 11, 20, 30, and 40, which differs substantially from the design of transceiver unit 1 according to FIG. 1 through the position and number of additional switches R₁ to R₄ or R′₁ to R′₄. Components having the same function are marked with the same reference numbers as in FIG. 1.

[0020] When several transceiver units 11, 20, 30, and 40 according to FIG. 2 are used, cross-echo analysis is possible with the aid of stochastic coding using switches R₁ to R₄, corresponding to switch R₁ under FIG. 1, if the stochastic pulse sequences of the transmission signal at each output 4 are known to the respective receiver branch at input 6. In the exemplary embodiment according to FIG. 2 the pulse actuation for each transmission window via switch 3 is enabled or disabled directly through switch R₁ for unit 11 to switch R₄ for unit 40, depending on the stochastic pulse sequence of the transmission signal of the respective units 11 to 40.

[0021] Pulses from all transmitting windows are supposed to be received in the receiver branch of the respective units 11 to 40. Hence the receiving window in each unit 11 to 40 must be opened upstream from mixer 7 and low-pass filter 9 with each of the pulses that is transmitted. The correlation of each stochastic transmission sequence of one of the units thus takes place only downstream from mixer 7. Here the signal received at each input 6, after filtering and absolute-value generation in low-pass filter 9, is switched or not switched to the respective accumulators A₁ (here 12 to 15 in unit 11) in the inter-pulse period involved, as a function of the random values 0 or 1 of switch R₁ (R₁ to R₄) of the respective transmitter branch of unit 1 (11 to 40).

[0022] For this purpose, the switch states of switches R₁ to R₄ are sent to each of units 11 to 40 via appropriate inputs to FIFO buffers 16 to 19 with which the switch states are delayed by a number of periods of pulse generator 5 that can be set in each case through which various distance ranges can be analyzed. The output voltage at each accumulator A₁ differs depending on whether the switch pulse sequence was correlated with the reception pulse sequence or not. For n transmitters one thus obtains n² accumulators A_(i,j)

[0023] (i,j =1 . . . n corresponding to the accumulator for transmitter j in receiver branch i). In this process accumulators A_(ii) receive the echoes of their own pulses and accumulators A_(i, j≠1) receive the cross echoes.

[0024] Since the propagation of the radar waves is reciprocal, E(A_(ij))=E(A_(ji)) applies to the expected values, i.e., each two cross-echo accumulators A_(ij),A_(ji) contains the same signal portion; only their noise portions differ. This circumstance can be used in a subsequent refinement for averaging the two accumulator units to reduce the noise further or to reduce the computing and hardware expenditure, since only one accumulator is implemented and analyzed. 

What is claimed is:
 1. A sensor array with a pulse echo radar system wherein a carrier signal of a microwave transmitter oscillator (2) is transmitted in the form of pulses with a specified pulse repetition frequency in a transmission window, reflected from a target object, and the position of the target object can be estimated in a mixer (7) from the time of transmission of the pulse to the time of arrival of the reflected radiation, and wherein the pulses can be transmitted by a random switch (R₁, R₁ to R₄) with a predefined probability, the stochastic pulse sequences of the transmission window in the receiver branch (7, 9, 10) being known.
 2. The sensor array according to claim 1 wherein an array is constructed from a plurality of transmitter and receiver units (11, 20, 30, 40) having switches (R₁, R₂, R₃, R₄) in which the stochastic pulse sequences of each transmission window in each receiver branch (7, 9, 10) are known and the transmitter and receiver units (11, 20, 30, 40) are linked with one another in such a way that the stochastic pulse sequences of each transmission window can be analyzed separately in each of the receiver branches (7, 9, 10).
 3. The sensor array according to claim 2 wherein the respective transmission window can be enabled via the switches (R₁, R₂, R₃, R₄) as a function of the stochastic pulse train of the respective transmission signal and a receiving window for all pulses that have been transmitted is opened in each receiver branch (7, 9, 10) via further switches (R′₁, R′₂, R′₃, R′₄) , and a correlation of each of the pulses associated with a transmission window with each of the respective pulses received can be carried out after a mixing of the signals received with the carrier signal in each of the respective mixers (7) of the transmitter and receiver units (11, 20, 30, 40).
 4. A sensor array according to claim 3 wherein in each of the transmitter and receiver units (11, 20, 30, 40) there is a FIFO buffer controlled by the switches (R₁, R₂, R₃, R₄) in the transmitter branch of each transmitter and receiver unit (11, 20, 30, 40) with which the respective switching signal for each of the switches (R₁, R₂, R₃, R₄) can be delayed by a number of periods of the pulse generator (5) that can be set in each case. 